When recovering samples using hollow stem augers, the normal procedure has been to remove a closure at the bottom of the hollow stem using a string of rods which are then withdrawn and used to lower a sampling tube or testing tool to the now open bottom of the stem. The tool is then withdrawn again and the closure replaced, again using the string of rods. Particularly with a deep bore, this procedure is extremely laborious and time consuming, and may require to be repeated at quite frequent intervals.
For this reason proposals have been made to provide closure means at the bottom of the stem which can remain in situ during sampling and testing operations. Examples of such proposals are contained in U.S. Pat. No. 3,095,051 issued June 25, 1963 to Robinsky et al, and Canadian Pat. No. 711,139 issued June 8, 1965 to Robinsky. In the first of these patents, the bottom of the auger tube is closed by a valve formed by two spring leaves extending from the bottom of the tube in a cusp-like formation. A sampling tube can be thrust between the valve members formed by these leaves so as to penetrate the soil beneath, and the leaves will spring back into place so as to close the auger stem when the tube is withdrawn. In the second patent, the bottom of the tube is closed by shaped rigid hinged valve members spring urged towards a closed condition. These members also are arranged to be thrust apart by a sampling tube.
With these arrangements it is necessary for the flights of the auger to project forwardly of the spring or valve members, which reduces the support available to the forward portions of the flights and weakens the structure of the auger. Moreover the spring or other valve members are somewhat vulnerable to damage. The valve arrangement increases the cost of the auger, thus providing a substantial financial burden if a range of augers are required to suit different soil conditions, or if frequent repair or replacement is required. It is known to provide earth boring augers with detachable teeth at their leading ends which can be exchanged or replaced as necessary, but these teeth usually require a distribution such that they cannot solely be supported by the leading ends of the flights. In the valved hollow stem augers considered above, the stem of the auger must stop short of the valve and is thus not available to support detachable teeth.
A powerful spring action must be provided by the spring leaves or applied to the valve members to bias them to a closed position, so as to ensure the avoidance of unwanted opening during boring and assured closing when a sampling tube is withdrawn. This power spring action means that considerable force is required to pass a sampling tube or other tool through the closure, and once so passed, it is gripped quite tightly by the spring leaves or valve members. One reason for using a hollow stem auger is to permit tests of the ground being bored by for example determining its resistance to rotation or penetration of test implements, and the gripping action of the spring leaves or other valve members interferes with such tests and prevents the obtaining of meaningful results. The present invention is directed to overcoming these problems.